POTABLE WATERS 3
Castile soap in 800 grams of 90% alcohol, filtering and adding half a
litre of distilled water to the filtrate.
(3) Burette containing about 5 c.c., the volume occupied by 2-4 c.c.
being divided into twenty-two equal parts, each representing one degree
of hardness ; above the zero is another mark up to which the burette must
be filled with soap solution and which represents the small quantity of
the liquid necessary in each case to produce froth, but not taken account
of in the calculation.
(4) Bottle with a ground-in stopper and marks indicating the volumes,
10, 20, 30 and 40 c.c.
The titre of the soap solution is first ascertained : into the bottle are
poured 40 c.c. of the calcium chloride solution and into this the soap solution
is gradually dropped from the burette (filled to the division above the zero
point) ; at intervals the bottle is closed and vigorously shaken up and
down. When such shaking produces a froth 5-6 mm. high at the surface
of the liquid and this persists for at least 5 minutes, addition of the soap
solution is discontinued. The soap solution is normal when the froth-
production requires 22 divisions of the burette (in addition to that above
the zero, which makes 23 in all). If less is required, the soap solution
must be diluted with water, and if more, soap must be added.
When 40 c.c. of calcium chloride •solution correspond exactly with 22
divisions of the soap solution, the total hardness of a water may be deter-
mined as above, 40 c.c. of the water being titrated with the soap solution
until a froth persistent for 5 minutes is obtained; the number of divisions
of the burette, calculating from zero, represents the number of French
degrees of hardness. One French degree corresponds with i gram of CaC03
per 100 litres ; thus, a water with 10 degrees of hardness contains 10 grams of
calcium and magnesium salts, calculated as calcium carbonate, per 100 litres.
If the soap clots during the test, the water probably contains more
than 30 degrees of hardness. In such case, the estimation is repeated
with 20, 10, or even 5 c.c. of the water, in fact with such a quantity that,
when a persistent froth is formed, the liquid is opalescent but without
clots ; the volume of water used is diluted to 40 c.c. with distilled water.
The dilution is, of course, allowed for in calculating the hardness.
To determine the 'permanent hardness, 100 c.c. of water are boiled in a
dish or flask for 20-30 minutes and, when cold, made up to the original
volume with distilled water and filtered; the hardness of the filtrate is
determined as above.
The temporary hardness is given by the difference between the total and
the permanent hardness.
(b) CLARK'S METHOD, modified by Faisst and Knauss.
This requires :
(1) An ordinary 50 c.c. burette.
(2) A bottle holding about 200 c.c. with a ground stopper and marked
at loo c.c.
(3) A solution of crystallised barium chloride containing o<523 gram,
of BaCla, 2H20 per litre.., _ .^ia>